1. The Field of the Invention
The invention relates to the field of optical amplifiers. More specifically, the invention relates to systems and methods for combining optical pumping sources for use in optical amplifiers.
2. The Relevant Technology
In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through a fiber-optic cable using light emitting diodes or lasers. Light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber-optic cable as can occur with electronic signals in wire-based systems. Light signals also can be conducted over greater distances without the signal loss typically associated with electronic signals on wire-based systems.
While signal loss in a fiber-optic cable is less than that in wire-based systems, there is nonetheless some signal loss over the long transmission distances that light signals are transmitted. To compensate for the signal loss, optical amplifiers are used. Two common optical amplifiers are Raman amplifiers and Erbium Doped Fiber Amplifiers (EDFAs). Both of these amplifiers use characteristics of fiber-optic cables to amplify light signals.
The amplifier pumps light on the fiber-optic cable that is at a different frequency than the light signal that is to be amplified. Energy from the light that is pumped on to the fiber-optic cable is transferred to the light signal due to the characteristics of the fiber-optic cable. Optical amplifiers use optical pumps to generate the light that is pumped into the fiber-optic cable. Optical pumps, however, are expensive. Although the price of a low-power optical pump is relatively low, high-power optical pumps are substantially higher priced.
When light is pumped into an optical system, the gain provided by the pumped light is dependent on the polarization of the light that is pumped into the optical system. If the polarization of the pumped light source fluctuates, the gain may fluctuate. To achieve consistent gain, a pumping source that provides a beam that is the combination of two orthogonally polarized beams with equal power in each beam is desired.
For this reason, polarization beam combiners (PBCs) are widely used in Raman amplifiers and EDFAs. They provide a simple way to combine two optical pumping sources that have perpendicular polarization directions and equal power into a single beam. They also provide even polarization distribution in the combined pumping beam in two orthogonal directions to minimize gain that is dependent on the pumping beam's state of polarization.
Commercially available PBCs are made using two different approaches: micro-optics and fused fiber. A micro-optic PBC, shown in FIG. 1A, is a PBC associated with fiber coupling devices, such as collimators including an optical lens and a fiber pigtail. A PBC can be made in several different ways, including Wollaston, Nicol, Rochon, Glan-Thompson, or Glan-Taylor prisms, using thin-film coatings on Right Angle Prisms (RAPs), or a single piece of birefringent crystal. A PBC made of high quality birefringence material tends to have better optical performance, such as a higher polarization extinction ratio, than dielectric-coating-based devices, yielding lower combining loss and higher power handling. Typical birefringent materials include Calcite, YVO4, Rutile, LiNbO3 and other single crystalline materials. A fused fiber PBC is simply a polarization maintaining (PM) fiber 2×2 fused fiber bi-conic coupler as shown in FIG. 1B, for example. A fused fiber PBC has a simple structure as well as low loss at the center wavelength and potential for low cost.
Both of these devices have limitations in practical applications. Multiple-wavelength pumping is often used to obtain wide and flat optical gains in the light signal bandwidth. Typically, two or three different wavelengths are used by the pump. These wavelengths fall about 20 nm apart and can cover a complete light signal bandwidth of at least 60 nm. To serve this wide bandwidth requirement, devices used for the pumping module should have flat performance response over the wavelength range. However, fused fiber devices show a 0.4 dB combining loss variance over a 60 nm wavelength bandwidth. Furthermore, it can be difficult to get equal combining efficiency for each input beam.
Another concern is controlling optical back reflections into the pumps. Most Raman pumps include multiple pump lasers. It is necessary to control the optical back reflection in order to stabilize the output of each laser and protect these lasers from being damaged. One common way to reduce back reflection is to employ optical isolators. The isolators are typically either in-line isolators, which can be fiber spliced into the optical path, or free space isolators used inside the pump laser module.
Many commercially available pump laser modules typically use fiber Bragg gratings (FBGs) to stabilize the pumping wavelengths. The FBGs are configured to reflect different wavelengths of light at different points in the optical path to compensate for the different speeds at which different frequencies of light travel in a fiber-optic cable. Free space isolators would block the reflections if placed in between the laser and the FBG, and thus cannot be used in such applications. Laser pumps thus require multiple external in-line isolators, which increases the cost and size of the pump.
Accordingly, it is desirable to integrate as many of the components of a pump as possible. This would serve to reduce component count and improve manufacturability, yielding improved optical performance in a smaller and less expensive module.